This invention relates to start up, shut down or other operation of an electrochemical cell array, such as a solid oxide electrolyte fuel cell array or an electrolyzer array, when the cell temperature is below the 800.degree. C. to 1200.degree. C. normal operating temperature of the array, to prevent cracking of the air electrode at such lower temperatures.
High temperature, solid oxide electrolyte electrochemical cell configurations of interior tubular air electrodes and exterior tubular fuel electrodes, with a solid oxide electrolyte therebetween, are well known in the art, and are taught in U.S. Pat. No. 4,395,468 (Isenberg). The air electrode of the fuel cell can be comprised of doped or undoped oxides or mixtures of oxides in the Perovskite family, such as LaMnO.sub.3, CaMnO.sub.3, LaNiO.sub.3, LaCoO.sub.3, LaCrO.sub.3, and the like. The usual air electrode material is LaMnO.sub.3 doped with Ca or Sr, as a substitute for part of the La. In all cases, the air electrode must be porous, that is, have from 20% to 40% porosity (60% to 80% of theoretical density) in order to be operable in the fuel cell. The air electrode can be supported by a porous, tubular support of stabilized zirconia, usually yttria stabilized zirconia.
The cells are usually operated in a square packed array of rows and columns with series-parallel electrical connections. These arrays are operated in a fuel cell generator, such as shown in U.S. Pat. No. 4,395,468 (Isenberg), where preheated air at 800.degree. C. to 900.degree. C. is fed within the fuel cells in the generating chamber to contact the air electrode and preheated fuel is fed around the periphery of the fuel cells in the generating chamber to contact the exterior fuel electrode at about 1000.degree. C. The cells can also be operated in an electrolyzer mode where electricity is put into the cell steam is fed to contact the exterior electrode, and O.sub.2 is generated within the cell.
It has been found that air electrode and air electrode porous support tube assemblies sometimes crack during start-up or shut-down thermal cycling and prolonged isothermal exposure in the temperature range of 550.degree. C. to 800.degree. C. What is needed is a process which can be used to operate such an electrochemical cell arrays in an electrochemical cell generator during such low temperature cycles. It is one of the objects of this invention to provide such a method of operation.